The objective of this study is to identify potential problems of and increase confidence in the operational calibration of the water vapor channel of the Meteosat Visible and Infrared Radiation Imager (MVIRI). First, the operational vicarious calibration method of the MVIRI water vapor channel is analyzed. This is done through a comparison of the operational calibration coefficient with a calibration coefficient derived from collocated satellite observations taken by water-vapor-sensitive channel 12 of the High Resolution Infared Sounder (HIRS) on NOAA 14. The analysis suggests a relatively large difference between the two calibration coefficients, the operational calibration coefficient being 10--15% larger than the one from the intersatellite calibration. Second, no such difference is found to exist between the MVIRI operational calibration coefficient and an intersatellite calibration coefficient derived from collocated GOES 8 water-vapor-sensitive channel 3 observations. Third, it is shown that the operational calibration coefficient for the MVIRI water vapor channel on Meteosat 7 is consistent with its blackbody calibration. At the present stage of the research no definite conclusions can be made. The results support the previously documented accuracy of 5% for the operational calibration coefficient of the MVIRI water vapor channel. ¿ 2001 American Geophysical Union